1. Field of the Invention
The present invention relates to multiphase drilling systems and methods.
2. Description of the Related Art
Drilling a wellbore typically requires circulating a drilling fluid to flush the bore of cuttings produced by action of a rotating drill bit. The drilling fluid may be pumped down the well inside the drill string and through the bit and jetted into the cutting face where it assists in penetrating the fractures created by the bit, lifting the bit fractured chips by penetrating the fractures and hydraulically lifting the chips into the circulating fluid stream. The drilling fluid then carries the chips up the lower annulus formed between an outer surface of the drill string and a wall of the wellbore. The drilling fluid and chips or returns continue up the upper annulus formed between a casing or lining and the drill string and to the surface where the chips are separated from the fluid. The cleaned fluid is then reintroduced to the well completing the circulation cycle. The drilling fluid may also cool the drill bit and support the wall of the wellbore.
Deploying a drilling rig to a wellsite is an expensive task. Oil and gas companies are constantly searching for ways to reduce time spent by the drilling rig at the wellsite. The rig time includes time spent drilling/tripping and non-productive time. The time spent drilling may be reduced by increasing a rate of penetration (ROP) of the drill bit through the rock formations, especially non-productive formations between the surface and the deeper hydrocarbon-bearing formations.
Primary factors which govern ROP include: bit type, weight on bit (WOB), rotary speed of the bit, formation characteristics, and a bottom hole pressure (BHP) exerted by the returns on the formation being drilled. The BHP includes a static component generated by the hydrostatic fluid column weight and a dynamic component generated by hydraulic drag due to naturally occurring resistance to flow through the annulus. The influence of BHP on the ROP is often discussed in terms of chip hold down effect. An increase in BHP tends to compact the rock formation being drilled, artificially increasing the shear strength of the rock and tending to hold rock chips created by the bit in place (chip hold down effect) thereby forcing re-drilling/re-fracturing of previously drilled rock and reducing the ROP. An increase in BHP also increases downtime by shortening the life of the bit, thereby requiring more frequent replacement or tripping of the drill bit.
To increase the ROP, drillers in many hard rock drilling areas have turned to using air for drilling fluid. As compared to conventional drilling mud, typically oil or water based, the density is reduced by orders of magnitude, thereby greatly reducing BHP, improving ROP and extending the life of the drill bit. However, air drilling is limited to select geological formations, which are consolidated with minimal water influx and do not produce hydrogen sulfide. Therefore, there exists a need in the art for a method of drilling a wellbore that increases ROP, improves the life of the drill bit, and does not suffer from the limitations of air drilling.